The invention relates to a method and structure for assembling a scanning unit in a position measuring instrument.
In position measuring instruments a scale is scanned by a scanning unit. The scanning unit generates position-dependent electrical scanning signals which are delivered to tracking electronics, such as a counter or a numerical controller to determine the amount of movement of the scale with reference to the scanning unit. The quality of the scanning signals depends on the spacing between the scanning unit and the scale which makes it necessary to mount the scanning unit at a precisely defined spacing relative to the scale.
U.S. Pat. No. 4,663,851 describes a position measuring instrument including a graduation unit and a scanning unit. The graduation unit has a centering flange, a scale and a graduation located on the scale. The scanning unit has a scanning component which includes a scanning graduation and a stop. During assembly, the scanning unit is positioned radially with respect to the scale by positioning the stop against the centering flange. The scanning component may then be moved along a guide towards the scale. As the scanning component is moved towards the scale the stop no longer makes contact with the centering flange. A spacing foil, removably sandwiched between the scale and the scanning graduation, is used to position the scanning graduation at a predetermined distance from the scale. A locking means is then used to prevent further movement of the scanning component along the guide. A disadvantage with such a position measuring instrument is the necessity of using a separate foil as a spacer.
U.S. Pat. No. 4,794,250 discloses a position measuring instrument and, more particularly, a rotary angle encoder that has no bearing of its own. Such rotary angle encoders are known as built-in rotary angle encoders. The relative position of the scanning unit and a scale is not defined until the scanning unit and scale are mounted on the objects to be measured. To simplify the mounting, a fixation element is integrated with the rotary angle encoder and is in contact with a hub of the scale and defines the position of the scale during the mounting. Once the scale has been mounted on one of the objects and the scanning unit has been mounted on the other object, the fixation element is removed from the hub. A disadvantage with such a position measuring instrument is the necessity of using a separate element, the fixation element, in order to position the scanning unit. Another disadvantage with this system is that the scale cannot be fastened independently of the housing and the fixation element on the shaft. At assembly the scale already has a clear association with the fixation element and the scanning unit.
It is desirable to provide a position measuring instrument that is simple in design and with which the requisite spacing between the scanning unit and the scale can be adjusted easily. It is also desirable to provide a position measuring instrument in which the scale can be fastened independently of the further components of the measuring device thereby simplifying assembly of the instrument. It is also desirable to provide a method that assures the simple and correct mounting of a scanning unit in a position measuring instrument.
An advantage of the invention is that the correct spacing required between the scanning unit and scale is defined by the components of the scanning unit itself without the need for additional components.